Buttondown Documentation
Buttondown has the ability to clean up spammy or unactivated email addresses from your subscriber list.
To turn the setting on, go to Settings > Subscribing.
Notably, this does not clean up subscribers who haven't opened your emails or clicked links in your emails recently.
We generally recommend against this, since click and open tracking is blocked by some email clients, making it look like these subscribers haven't opened your emails. Cleaning up these subscribers would remove actual readers from your subscriber list.